Improved techniques for the purification of retroviral gene products have been developed. Purified proteins of primate retroviruses have been employed in sensitive immunoassays for the detection of retroviral antigens and antibodies. Extensive screening of sera and tissues obtained from humans for evidence of retroviral infection is now feasible. Sera obtained from 2000 humans, many of whom had exposure to FeLV, failed to reveal immunologic evidence of viral infection. Thus, the risk to humans from this oncogenic virus appears to be minimal. Screening of tissues from subhuman primates and from man for immunologic evidence of endogenous viruses related to known retroviruses is in progress. The recent development of broader and more sensitive immunoassays may be important in these studies. Studies by others have indicated a high incidence of antibodies in humans to known primate retroviruses. These studies have employed radioactively labelled whole virus as opposed to purified viral proteins. It has been possible to show that the reactivity detected in those studies results from technical artifact. This finding emphasizes the need to employ highly purified viral protein probes in systems designed to assess the prevalence of retrovirus infection in man.